The invention relates to hearing aids wherein a transistor amplifier is disposed between the microphone and the earphone or other output transducer, and comprises a stage containing a frequency-determining network. Circuits of this type are used for the purpose of being able to adapt the effectiveness of hearing aids to the nature of the hearing difficulty (compare e.g., Jour. Acous. Soc. Am., Vol. 11, (1940) pp. 406 through 419).
Accordingly, a great many attempts have already been made to adapt to the frequency response which, in the case of a hearing-impaired individual, is detected as residual sensitivity. To this end, microphones have been employed in the hearing aid which have a rising frequency response, and, in addition, the coupling capacitors between the individual stages have been reduced to such an extent that a treble boost characteristic results. Moreover, sound frequency filters have been additionally used in which the input resistance of an amplifier stage in the hearing aid is reduced in combined action with the aforementioned coupling capacitor. However, these measures left much to be desired, because the effectiveness of sound filters of this type did not permit an optimum compensation of a majority of the treble losses. The sound filter should be effective in a single stage with a 12 dB per octave rising response.
From the German offengungsschift No. 2316939 e.g., an electric hearing aid circuit is known wherein the transmitted frequency range is divided into separate regulatable component regions. An improved adaptation to the frequency-dependency of reduced hearing ability is to be achieved thereby. However, for arrangements of this type, it is necessary to be able to vary the region of rising response (rising flank) of the filter frequency response characteristic within wider boundaries than hitherto. Doubtlessly, this necessitates a great outlay which is of particular disadvantage if it is desirable to construct small hearing aids to be worn on the head wherein only a small amount of installation space is available. In addition, a further disadvantage is that, due to fluctuations in the battery voltage, stabilization measures requiring great outlay become necessary for the continuous variability of the rising flank of the filter frequency response.